Group A meningococci cause massive epidemics of meningitis and sepsis in sub-Saharan Africa. Compared to other bacterial polysaccharides (PS), group A has a number of unusual properties including being highly immunogenic in infants, and priming for booster antibody responses. Also, depending on the age of the person, or antigenic stimulus (natural exposure to group A or cross-reacting organisms, or conjugated vs. unconjugated PS vaccination), the PS can elicit bactericidal or non-bactericidal group A anticapsular antibodies. The role of non-bactericidal group A anticapsular antibodies in protection against group A disease is unknown, and the molecular basis for differences in antibody functional activity are poorly understood. Our hypothesis is that differences in antibody avidity and/or fine antigenic specificity, dictated by the structure of the antibody paratope, underlie these disparities in antibody functional activity. In this proposal we will characterize naturally acquired and vaccine-induced group A anticapsular antibodies from persons of different ages living in North America or sub-Saharan Africa, two areas of the world with vastly different risks of exposure to group A meningococci. Passive antibody protective activity will be measured in an animal model of group A bacteremia that will be developed. To define the V region genes utilized by the human antibody response to group A PS, and to determine the extent of hypermutation, we will perform combinatorial repertoire cloning and expression library analyses of group A PS-specific Fab fragments. This approach will be complemented by limited amino acid sequencing of VH and VL regions of clonally purified anticapsular antibodies and determination of V region genes by mass fingerprint analysis by MALDI-TOF mass spectroscopy of H and L chains separated by 2D gels. Together, these studies will elucidate the molecular basis by which human antibodies recognize group A PS, and will identify the mechanisms underlying the age- and vaccine-related disparities in antibody protective activity. The results may lead to establishment of more reliable surrogates of protective immunity for assessment of the efficacy of new group A conjugate vaccines being developed for elimination of epidemic meningococcal disease in sub-Saharan Africa. Our proposed studies also will increase our knowledge of human antibody recognition of bacterial PS antigens in general, and explain why some anticapsular antibodies confer protection against encapsulated bacteria, while others do not.